For correct operation, carburetors require the maintenance of a predetermined fuel supply pressure. A fuel level regulator usually serves for this purpose, this regulator comprising a level sensing device and a feed valve which is automatically actuated by this device. The level sensing device is usually formed by a float in a comparatively large float chamber. The float acts upon the feed valve which is usually constructed as a needle valve and is often known as a float needle valve, by mechanical transmission members in such a way that the valve is closed as the fuel level in the chamber rises and is opened as the fuel level falls.
This form of regulator has proved satisfactory in normal operation of internal combustion engines, on account of its simplicity. It makes it practically impossible, however, accurately to determine the instantaneous fuel consumption of an engine. The reason for this is that the volume of fuel in the float housing is subject to relatively large fluctuations. These fluctuations occur firstly as a result of "float oscillation," which is produced, especially at low rates of flow, by the friction on the float and an adhesion force at the float needle valve. A further reason for the fluctuations in the volume of fuel in the float chamber lies in the splashing around of the fuel surface resulting from vibrations of the engine or movement of the vehicle in which the engine is mounted. This causes drastic movements of the float with corresponding, undesired opening and closing of the float needle valve.
The float chamber acts as a buffer or store for the different rates of flow of fuel that occur, so that the instantaneous fuel flow into the carburetor is not the same as the fuel outflow from the carburetor, that is to say the instantaneous consumption by the engine. The inflow and outflow are however the same over a fairly long period, so that average consumptions can be accurately measured and calculated. However, in a test run, for example, the accurate and continuous determination of the instantaneous fuel consumption is of interest, in order to find out as rapidly as possible the related effects of changes as the carburetor, for example of the jet equipment, and thereby to shorten the test time. Furthermore, when testing carburetors after production, considerable reductions in test times, that is substantial savings, can be achieved. The need for an indication of the instantaneous fuel consumption is not limited, however, to test the trial running, but exists in daily operation on account of high fuel prices and energy scarcity among vehicle drivers in order for the drivers to be able to adapt their driving techniques to achieve maximum economy. An acceptable price is, however, a necessary requirement for the introduction of such a fuel consumption measuring device as an additional item of equipment. The device must therefore be as simple as possible and be economical to manufacture.